From the people who are not so stable financially. The "free" service is the gateway for many people. Eventually plenty of them will default or use services which do make money for Klarna.

You'd be shocked to see for example how many people pay just the interest on their credit card because the "free" service (when you repay everything monthly and pay nothing extra) encouraged them to use it so much they overburdened themselves. I think Klarna operates along the same lines.